1. Field of the Invention
The present invention relates generally to base materials for fabricating a carrier, a substrate and a case for use in an assembly of electronic components.
2. Description of the Background Art
A base material for mounting electronic components should have physical properties suitable for maintaining performance and reliability of an electronic assembly. In particular, the base material preferably has a coefficient of thermal expansion (referred to as CTE hereinafter) as close as to CTE (6-8.times.10.sup.-6 /.degree. C.) of a semiconductor as an electronic component and a semiconductor packaging material. Furthermore, the base material is desirably light-weight.
Aluminum and an aluminum alloy having a low density (about 2.7 g/cm.sup.3) have been conventionally used as suitable materials in view of weight. These metallic materials, however, have a CTE (23.times.10.sup.-6 /.degree. C.) higher than those of semiconductor materials. As a result, thermal stress induced between a semiconductor device and a case made from the metallic materials might cause cracks or debonding in the joint portion thereof, resulting in defective electric connection of the semiconductor device, detachment of the components, disconnection of wirings and the like. Electronic assemblies using such metallic materials are not satisfactorily reliable over operating temperature extremes.
Kovar, a high-nickel steel, has typically been used to overcome these shortcomings. Kovar effectively prevents generation of thermal stress because of its CTE (about 6.times.10.sup.-6 /.degree. C.) lower than that of aluminum. However, high density (8.0 g/cm.sup.3) of Kovar leads to an increase in weight of electronic assemblies.
Under the above-described circumstances, metal-matrix composites (referred to as MMC hereinafter) consisting of an aluminum matrix alloy reinforced with silicon carbide particles have been developed for microwave packaging application and the like (Electronic Packaging and Production, August, 1987, p. 27-29; Proceedings of the 1st International SAMPE Electronics Conference Jun. 23-25, 1987, p. 452-462; Proceedings of the 3rd International SAMPE Electronics Conference Jun. 20-22, 1989, p. 1068-1077.). MMC is 60% or more lighter than Kovar and have a CTE lower than that of aluminum or an aluminum alloy. However, silicon carbide contained in MMC makes mechanical processing more difficult, thereby increasing cost therefor. In addition, with a density of about 2.9 g/cm.sup.3, the silicon carbide increases weight of the assembly more than by aluminum or an alloy thereof.
Graphite fiber reinforced aluminum (Grf/Al) with fibers arranged in a crossplied manner has been developed for microwave devices (Proceedings of the 1st International SAMPE Electronics Conference Jun. 23-25, 1987, p. 452-462.). Gr.sub.f /Al has a low CTE and a low density. However, G.sub.f /Al has low strength in a vertical direction to the piling direction of the fibers. In mechanical processing, crossplied fibers peel off in many cases. A processing rate should be reduced to prevent peeling off. When peeling off occurs, the peeled surface should be abraded. An arrangement of graphite fibers in a crossplied manner requires span fibers to be knitted to fabricate cloths which are to be piled. Such process results in low productivity of base materials. In addition, with cloths piled with a particularly low volume fraction, it is not possible to obtain a homogeneous composite material because of difficulty in matching the volume fractions in the entire pile.